The thermal stability of a series of cobalt and nickel molybdates (AMoO4·nH2O, α-AMoO4, and β-AMoO4; A = Co or Ni) was examined using synchrotron-based time-resolved X-ray powder diffraction (XRD). The results of X-ray absorption near-edge spectroscopy (XANES) indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the α-phases to tetrahedral in the β-phases and hydrates. Upon heating of AMoO4·nH2O, evolution of gaseous water was seen at two different temperature ranges:  100−200 °C for reversibly bound H2O; 200−400 °C for H2O from the crystal structure. The results of time-resolved XRD show a direct transformation of the hydrates into the β-AMoO4 compounds (following a kinetics of first order) without any intermediate phase. This is probably facilitated by the similarities that AMoO4·nH2O and β-AMoO4 have in their structural and electronic properties. The XRD experiments show that the α-AMoO4 → β-AMoO4 transitions occur at much higher temperatures than the hydrate → β-AMoO4 transformations (ΔT ≈ 150 °C in CoMoO4 and 280 °C in NiMoO4). The activation energy for the α-NiMoO4 → β-NiMoO4 transition is ∼40 kcal/mol larger than that for the NiMoO4·nH2O → β-NiMoO4 + nH2O reaction. The larger activation energy reflects the change in the coordination of Mo (O h → T d ) that occurs during the α → β transition. The K- and L-edges of Co in XANES spectra indicate that there are no big variations in the electronic properties of this metal when comparing CoMoO4·nH2O, α-CoMoO4, and β-CoMoO4. The same is valid for the electronic properties of Ni in the nickel molybdates. In contrast, the L2,3-edges of Mo show large changes in the splitting of the Mo 4d orbitals as the coordination of this metal varies from octahedral (α-phases) to tetrahedral (β-phases and hydrates). The features near the threshold in the O K-edge spectra track very well the splitting of the Mo 4d orbitals in tetrahedral and octahedral fields and can be very useful for probing the local symmetry of Mo atoms in molybdenum oxides.

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The adsorption and dissociation of H2S and S2 on a series of oxide (Al2O3, Cr2O3, Cr3O4, Cu2O, ZnO) and metal/oxide (Cu/Al2O3, Cu/ZnO) surfaces have been studied using synchrotron-based high-resolution photoemission. H2S and S2 mainly interact with the metal centers of the oxides. At 300 K, H2S undergoes complete decomposition. The rate of decomposition on Al2O3 is much lower than those found on Cr3O4, Cr2O3, ZnO, and Cu2O. For these systems, the smaller the band gap in the oxide, the bigger its reactivity toward S-containing molecules. The results of ab initio SCF calculations for the adsorption of H2S, HS, and S on clusters that resemble the (0001) face of α-Al2O3, α-Cr2O3, and ZnO show that the S-containing species interact stronger with Cr or Zn than with Al centers. These theoretical results and the trends seen in the experimental data indicate that the reactivity of an oxide mainly depends on how well its bands mix with the orbitals of H2S or HS. The electrostatic interactions between the dipole of H2S and the ionic field generated by the charges in the oxide play only a secondary role in the adsorption process. Photoemission results show that the rate of adsorption of H2S and S2 on Cu/Al2O3 and Cu/ZnO surfaces is much faster than on the pure oxides. A simple model based on perturbation theory and orbital mixing is able to explain the effects of the band-gap size on the reactivity of an oxide and the behavior of metal/oxide surfaces in the presence of S-containing molecules.

The surface chemistry of SO2 on polycrystalline Sn, Pt(111), and a ( x )R30°-Sn/Pt(111) surface alloy has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent field calculations. Metallic tin has a large chemical affinity for SO2. At 100−150 K, SO2 disproportionates on polycrystalline tin forming multilayers of SO3 (2SO2,a → SOgas + SO3,a). At these low temperatures, the full dissociation of SO2 (SO2,a → Sa + 2Oa) is minimal. As the temperature is raised to 300 K, the SO3 decomposes, yielding SO4, S, and O on the surface. Pure tin exhibits a much higher reactivity toward SO2 than late transition metals (Ni, Pd, Pt, Cu, Ag, Au). In contrast, tin atoms in contact with Pt(111) interact weakly with SO2. A ( × )R30°-Sn/Pt(111) alloy is much less reactive toward SO2 than polycrystalline tin or clean Pt(111). At 100 K, SO2 adsorbs molecularly on ( × )R30°-Sn/Pt(111). Most of the adsorbed SO2 desorbs intact from the surface (250−300 K), whereas a small fraction dissociates into S and O. The drastic drop in reactivity when going from pure tin to the ( × )R30°-Sn/Pt(111) alloy can be attributed to a combination of ensemble and electronic effects. On the other hand, the low reactivity of the Pt sites in ( × )R30°-Sn/Pt(111) with respect to Pt(111) is a consequence of electronic effects. The Pt−Sn bond is complex, involving a Sn(5s,5p) → Pt(6s,6p) charge transfer and a Pt(5d) → Pt(6s,6p) rehybridization that localize electrons in the region between the metal centers. These phenomena reduce the electron donor ability of Pt and Sn, and both metals are not able to respond in an effective way to the presence of SO2. The Sn/Pt system illustrates how a redistribution of electrons that occurs in bimetallic bonding can be useful for the design of catalysts that have a remarkably low reactivity toward SO2 and for controlling sulfur poisoning.

Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science. We have examined the structural and electronic properties of NiMoO4 and MgMoO4 by means of synchrotron-based time-resolved x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and first-principles density functional theory (DFT) calculations. Nickel molybdate can exist in two phases (α and β). Mo is in a near tetrahedral environment in the β-phase, whereas in the α-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 Å). The results of DFT calculations indicate that the α-phase of NiMoO4 is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an α-NiMoO4-type phase is ∼13 kcal/mol less stable than β-MgMoO4. These trends in stability probably result from variations in the metal–metal repulsion within the α-phases of the compounds. For the α→β transition in NiMoO4, the DFT calculations predict an energy barrier of ∼50 kcal/mol. An apparent activation energy of ∼80 kcal/mol can be derived from the time-resolved XRD experiments. The degree of ionicity in MgMoO4 is larger than that in NiMoO4. The nickel molybdate displays a large density of states near the top of the valence band that is not observed in the magnesium molybdate. This makes NiMoO4 more chemically active than MgMoO4. A similar type of correlation is found between the electronic and chemical properties of NiMoO4, CoMoO4, and FeMoO4. The DFT results and Mo LII-edge XANES spectra show big differences in the splitting of the Mo 4d orbitals in the α- and β-phases of the molybdates. The line shape in the O K-edge essentially reflects the behavior seen for the 4d orbitals in the Mo LII-edge (i.e., mainly O 1s→Mo 4d electronic transitions). The Mo LII- and O K-edges in XANES can be very useful for probing the local symmetry of Mo atoms in mixed-metal oxides.